Operators of mobile communication networks, such as GSM/WCDMA operators, that install IMS (IP Multimedia Subsystem) networks and offer IMS services want to make these IMS services available also to their subscribers within the Circuit Switched Domain. Hereto, calls from and to this group of (GSM/WCDMA) subscribers need to be routed through the IMS network in order to reach a respective IMS service engine offering the respective service. This concept is also known as IMS Centralized Services (ICS). The IMS Centralized Services work item in 3rd Generation Partnership Program (3GPP) targets using the Circuit Switched network as access to IMS services, (see 3GPP Technical Specification TS 23.292) and is complemented by IMS Service Continuity (see 3GPP TS 23.237).
In parallel to the ICS trend the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS), supporting Evolved UTRAN (E-UTRAN) as new radio access network. Single Radio-Voice Call Continuity (SR-VCC) within said framework would allow for Handover of an IMS voice call from EPS to CS. This is what is currently supported by standards. It is envisioned that SR-VCC is combined with ICS for an IMS centralized solution supporting also SR-VCC from E-UTRAN to UTRAN/GERAN. In the following the terminology of EPS and UTRAN/GERAN CS will be used synonymously.
In FIG. 1 an exemplary architecture is displayed; an MSC Server which is enhanced for SR-VCC has either an ISUP (ISDN User Part) or an SIP (Session Initiate Protocol) interface. For the purpose of this application, it is assumed in the following that the enhanced MSC Server has a SIP interface.
Within said architecture a Handover of an IMS voice call from EPS/LTE towards GERAN/UTRAN CS is conceptually shown. In the lower part, the UE being connected via the E-UTRAN towards the IMS network via the Serving/PDN gateway is shown. This User Equipment is now to be handed over (as indicated by the arrow on the left side) towards the UTRAN/GERAN CS in which it would be connected to the IMS network via the Target UTRAN/GERAN and the MSC-Server. Other entities than the MME which will be detailed in the following, e.g. a SGSN (serving Gateway Signaling Node) and a HSS (Home Subscriber Server) are only shown for completeness but are not necessary to understand the invention.
FIG. 2 depicts an exemplary flowchart, illustrating a Handover from an LTE/EPS system (E-UTRAN) towards a UTRAN/GERAN CS system as exemplified in the architecture of FIG. 1.
Within said scheme due to some reasons, e.g., measurement reports as indicated in step 1 of FIG. 2, lead to decision in the source E-UTRAN/HSPA (lower portion of FIG. 1) to initiate a handover towards a target UTRAN/GERAN CS, i.e. the upper portion in FIG. 1. The relocation request is sent towards the MSC-Server which prepares the Target network (Target MSC, Target BSS) for the handover (steps 4 through 7). Thereafter in step 8, a transfer of the session is initiated towards the IMS. After step 8, the Service Centralization and Continuity Application Server (SCC AS) in the IMS (an SCC AS correlates Service Control Signaling and CS Control Signaling for a session in IMS) has performed the session transfer for speech media, i.e., the remote end within the IMS has been updated to send speech media to the MGW controlled by the MSC Server enhanced for SR-VCC and not longer over E-UTRAN to the User Equipment (UE).
While the eNode B as part of the Source E-UTRAN has decided in step 2 to perform a handover (decision to HO) which causes the Mobility Management Entity (MME, e.g., SGSN) to send a Forward Relocation Request towards the MSC Server), it is allowed that the eNode B part of the Source E-UTRAN may decide later that it is no longer required to perform handover, e.g., due to newer measurement reports. In such a case and in case of a Packet Switched (PS) handover, the eNode B would inform the MME accordingly (or the NodeB would inform the SGSN) and the PS handover procedure would be stopped and all bearers would remain in the EPS.
However, when the eNode B would decide to not require handover after the session transfer has been initiated by the MSC Server enhanced for SR-VCC (step 9), the UE would stay in E-UTRAN but the speech media would have been already sent to the target MSC Server enhanced for SR-VCC. Thereby the user would not be able to continue his phone call, since the media and the related SIP control signaling would be directed towards the target MSC Server/MGW and not via E-UTRAN.